Packing heat ... into reusable energy

May 1, 2008 OpenSystems Media

Excessive heat is a significant concern in any computer design. Though designers have implemented varying methods of heat dissipation, all share one common problem. Heat dissipation relies on the transferring medium’s thermal conductivity. Many ingenious contraptions have been invented to achieve faster and more efficient heat dissipation. But instead of throwing heat away, why not recycle it? John presents a potentially energy-salvaging thermal management strategy.

Ask most laptop or notebook computer users about their main annoyance, and their responses will likely have something to do with the heat their computers produce. Laptop and notebook computers use many of the same components as embedded computer designs – components such as the central and graphics processing units, Northbridges, Southbridges, and so on.

Components play a significant role in a thermally friendly computer design. Using such low-heat processors like the VIA C7 CPU help reduce overall heat[1]. But generally, as processors become more powerful, they generate more heat, making thermal management an important consideration.

Herein lies the problem: Computers require power to operate, and that power inevitably produces waste energy in the form of heat. If the accumulated heat within a system becomes too great, the system cannot operate properly. Designers’ objective, then, is to get rid of the generated heat and use it productively, if possible.

Current thermal management techniques

At present, most if not all thermal management methods concentrate on heat dissipation, either passive (for example, fanless heat sinks) or active (for example, fan-forced cooling or water cooling). An ideal thermal management system based on heat dissipation would dissipate all the generated heat. Though existing thermal management devices are adequate, they are limited in how much heat they can dissipate. Heat dissipation is only satisfactory until thermal equilibrium is reached.

How much heat needs to be dissipated? Generally, the amount of energy used is proportional to the amount of heat produced. So if a component uses 30 W of power over 60 seconds, it will output 1,800 joules of heat.

Q = Pt = VIt

In the equation above, Q represents heat in joules (J), P represents power in watts (W), t represents time in seconds (s), V represents voltage (V), and I represents current in amps (A). The delta T degree Celsius can be calculated using the following equation, where m represents mass in kilograms (kg) and c represents specific heat (J/gK).

ΔT = Q/mc = VIt/mc

A heat-sink system is based on transferring heat from the heat sink to ambient air. For a heat sink to properly work, thermal equilibrium must never be reached while the system is running (that is, ambient air must always be cooler than the heat sink when the system is on). Otherwise, once thermal equilibrium is reached, the heat will no longer transfer from the heat sink to ambient air.

Once thermal equilibrium is reached, the heat sink cannot do much to relieve the processor of its heat burdens. Even assuming that thermal equilibrium is never reached, it is evident that much energy is wasted. Such potential should not go unused. An alternative thermal management technique involves converting heat energy and storing it for later use.

Making heat productive

Converting heat energy to electricity is not a new idea. Because heat is infrared radiation and thus part of the electromagnetic spectrum, photovoltaic-based thermal management is possible[2]. The term thermophotovoltaic (TPV) has been used to describe photovoltaic devices for converting heat energy[3]. Currently, TPV devices are not practical enough to consider as primary power sources. However, it is important to remember that thermal management systems’ chief objective is heat removal, not power generation. Any power that is gleaned from the process is a bonus and not the main goal.

Photovoltaic devices require two layers of photosensitive material: one p-type layer and one n-type layer. The p-type comprises a material whose atoms have an extra electron that prevents the material from being completely nonconductive. The n-type consists of a material whose atoms lack one electron. When the p-type layer is exposed to light, the photons in the light source cause the extra electrons to be released, resulting in a flow of electricity (see Figure 1).

21
Figure 1

Implementation for a fully enclosed system

A thermal management device that recycles waste energy is ideal for airtight sealed systems. In such systems, passive cooling and fan-forced cooling have little if any effect. When waste energy is recyclable, rugged airtight systems can be designed without sacrificing performance. High-performance components generally produce more heat, but for a thermal management device fueled by heat, it is no longer a problem.

To implement the thermal management device, the chassis interior must be lined with TPV cells to absorb as much heat as possible. In addition, a double-sided TPV cell should be placed directly over the heat sink (see Figure 2). Having a double-sided design enables the TPV cell to draw heat not only from the heat sink but also from ambient air. The chassis interior wall is lined with single-sided TPV cells to capture any remaining ambient heat.

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Figure 2

This thermal management strategy is particularly suitable for portable applications or external applications where the environment may adversely affect the computer (for example, a street kiosk, GPS unit on a motorcycle, or navigation unit on marine craft). The technique also would work in partially enclosed systems such as fixed applications where the environment will not have adverse effects on the computer (for example, a kiosk inside a building).

Perpetuating battery life

By using TPV cells in thermal management devices and choosing the right components, designers can create winning low-heat systems. As photovoltaic cell technology advances, such designs may result in products with ridiculously long battery lives – and perhaps even a nearly perpetual energy source.

References

  1. "VIA C7-M ULV Processor," VIA Technologies website. 2007. VIA Technologies, Inc. 6 Dec, 2007.
  2. "Solar Cells that Harness Infrared Light," Environmental Science & Technology. 2 Mar. 2005. Konarka Technologies, Inc. 14 Nov. 2007
  3. "Thermo-photo-voltaic Cell Based on GaSb," TPV Network. 2007. 12 Nov. 2007

John Lin is a senior technical writer at VIA Technologies, Inc., based in Taipei, Taiwan. He has seven years of experience in the computer industry, with five-plus years at VIA. John graduated with a Bachelor’s degree in Business Economics from UCSB.

VIA Technologies, Inc.
+886-2-2218-5452
JohnLin@via.com.tw
www.via.com.tw

John Lin (Via Technologies)
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